High levels of sulphur in diesel fuel in some less developed automotive markets, notably large parts of China, are hampering efforts to meet newly targeted emission standards, even with the most modern engine technology. Arguably the greatest challenge relates to particulate matter (PM) limits.
Diesel particulate filters (DPFs) are now fitted almost universally on new compression-ignition engined trucks and buses in Europe, North America and Japan. Most are self regenerating – up to a point. That is to say they embody a catalyst, most often platinum, which stimulates oxidation of the filtered soot particles at regular exhaust temperatures. But where the load on an engine is insufficient to heat up the exhaust, bringing the catalyst up to its crucial light-off temperature (around 600 degrees C), most frequently under urban stop-start vehicle operating conditions, unoxidised carbon particles accumulate in the pores of the filter element.
High levels of sulphur in diesel fuel in some less developed automotive markets are hampering efforts to meet newly targeted emission standards, even with the most modern engine technology
Partial or complete blockage of the filter then ensues, with the possibility of it being damaged or destroyed, meaning costly replacement. That risk is minimised on many newer trucks and buses compliant with Euro V, Euro VI, EPA 2007, EPA 2010 and equivalent Japanese legislation, as well as some slightly older vehicles. Those incorporate either an in-cylinder post-injection phase or downstream fuel spray function within the manifold, triggered by sensors which detect imminent DPF clogging.
That extra fuel burn serves to raise exhaust system – and hence DPF catalyst – temperature to regeneration (i.e. soot oxidation) level. Though that ensures the required PM reduction, while keeping the DPF free of soot build-up, it clearly jeopardises overall fuel consumption, in an era when fuel costs loom large in transport operators’ budgets.
In any case, even where hard-working engines keep their exhausts hot enough to maintain active regeneration, the catalytic function can be marred by an excessive level of sulphur in the fuel, which can ‘poison’ and disable a platinum catalyst, sometimes irretrievably. It brings a major challenge in areas such as rural China, where diesel sulphur content can be as high as 1400 parts-per-million (ppm), compared with the 15ppm or lower now demanded in Europe and elsewhere.
A particular attraction of FBCs in low exhaust temperature applications is that such additive systems can be installed retrospectively on older, smokier vehicles
One answer to the challenge, as China prepares to implement Euro IV equivalent emission laws, looks to be the use of a fuel-borne catalyst (FBC) of the kind now being promoted ever more actively by additive specialists such as Infineum (owned jointly by Shell and Exxon), Lubrizol, Rhodia and Eminox. Infineum has just introduced an iron-based FBC intended for mixing directly into the fuel from a small additive tank on the vehicle. The FBC becomes incorporated into the exhaust PM and is claimed ‘to lower the soot burn-off temperature by over 150 degrees C’. It is said to be ‘highly tolerant of fuel sulphur’.
A particular attraction of FBCs in low exhaust temperature applications, for example in city buses, waste collection vehicles and even diesel taxicabs, where DPF clogging has proved a widespread problem, is that such additive systems can be installed retrospectively on older, smokier vehicles. Their successful adoption looks likely however to hinge, not surprisingly, on the litre-cost of the additive and the required dosage rate, which inevitably varies widely under different conditions.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
Alan Bunting has a background in engineering, and has been writing on commercial vehicle and powertrain related topics since the 1960s. He has been an Automotive World contributor since 1996.
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